type DescribleFunction = {
  description:string
  (someArg: number):boolean
}

function doSomething(fn:DescribleFunction){
  console.log(fn.description + 'returned' + fn(6));
}

function fn1(n:number){
  console.log(n);
  return  true
}

fn1.description = 'hello'

doSomething(fn1)